Presentation on theme: "Energy Lecture Slide 1 Energy Conservation. Energy can be thought of as the capacity for doing work Energy may be transformed from one type of energy."— Presentation transcript:
Energy can be thought of as the capacity for doing work Energy may be transformed from one type of energy to another. –Kinetic, potential, “heat”, work Energy can be neither created nor destroyed Energy Lecture Slide 2
Energy Lecture Slide 3 Energy Conservation The total energy of a system is constant. That is, the sum of the PE + KE + other types of energy = constant If PE decreases, KE increases … and vice versa
Pendulum Answers A: h = 0.306 m (6 J = 2 kg *9.8 m/s/s * h) B: h = 0.153 m (3 J = 2 kg *9.8 m/s/s * h) C: v = 1.73 m/s (3 J = 0.5 * 2 kg * v 2 ) D: h = 0 m (0 J = 2 kg * 9.8 m/s/s*h) E: v = 2.45 m/s (6 J = 0.5 * 2 kg * v 2 ) F: h = 0.306 m (6 J = 2 kg * 9.8 m/s/s * h) Energy Lecture Slide 7
Energy Lecture Slide 12 Simple Machines If no friction, then Work Input = Work Output Inclined plane Pulley Lever
Why Use a Machine? If there is no friction, –Work done on the machine (work input) is equal to the work done by the machine (work output) and … when there is friction, –work input > than work output So why do we use machines? Energy Lecture Slide 13
Energy Lecture Slide 14 Inclined Plane Output work = work done if you didn’t use the machine –Output work = (weight of load)(vertical height) Input work = work done using machine –Input work = (force exerted)(length of plane)
Inclined Plane Problem Energy Lecture Slide 15 What work would be done by the man if he lifted the block vertically upward 3m? How does this relate to the change in potential energy of the block? The mass of the block of ice is 50 kg.
Inclined Plane Problem Energy Lecture Slide 16 What work would be done by the man if he slid the block along the incline to the top? What force would the man have to use to move the block at a constant speed along the ramp? The mass of the block of ice is 50 kg.
Ramps & Energy Conservation What is the speed of the block at the bottom of each of the 5 m high ramps? Ignore friction. Use g = 10 m/s/s. Energy Lecture Slide 17
Energy Lecture Slide 18 Pulley Mechanical Advantage of Single Pulley is 1 Single pulley changes direction of force
Pulley Systems Energy Lecture Slide 19 Compound Pulleys can multiply force.
Window Washer The window washer has a mass of 75 kg and the platform plus the bucket and squeegee has a mass of 25 kg. What force does the washer have to exert to lift himself and the platform? Energy Lecture Slide 20 What length of rope will he have to pull in order to lift himself 12 m?
Energy Lecture Slide 22 Mechanical Advantage Ideal Mechanical Advantage
Energy Lecture Slide 23 Efficiency Because of friction, –work output < work input –Efficiency < 100% –Efficiency can also be found by
Energy Practice 1 Both the KE and PE of a block freely sliding down a ramp are shown below only at the bottom position in the sketch. Fill in the missing values for the other positions Energy Lecture Slide 24